Fibroblast Growth Factor 21 (FGF21) is a secreted polypeptide that belongs to a subfamily of Fibroblast Growth Factors (FGFs) that includes FGF19, FGF21, and FGF23 (Itoh et al., (2004) Trend Genet. 20:563-69). FGF21 is an atypical FGF in that it is heparin independent and functions as a hormone in the regulation of glucose, lipid, and energy metabolism.
It is highly expressed in liver and pancreas and is the only member of the FGF family to be primarily expressed in liver. Transgenic mice overexpressing FGF21 exhibit metabolic phenotypes of slow growth rate, low plasma glucose and triglyceride levels, and an absence of age-associated type 2 diabetes, islet hyperplasia, and obesity. Pharmacological administration of recombinant FGF21 protein in rodent and primate models results in normalized levels of plasma glucose, reduced triglyceride and cholesterol levels, and improved glucose tolerance and insulin sensitivity. In addition, FGF21 reduces body weight and body fat by increasing energy expenditure, physical activity, and metabolic rate. Experimental research provides support for the pharmacological administration of FGF21 for the treatment of type 2 diabetes, obesity, dyslipidemia, and other metabolic conditions or disorders in humans.
FGF21 is a liver derived endocrine hormone that stimulates glucose uptake in adipocytes and lipid homeostasis through the activation of its receptor. Interestingly, in addition to the canonical FGF receptor, the FGF21 receptor also comprises the membrane associated β-Klotho as an essential cofactor. Activation of the FGF21 receptor leads to multiple effects on a variety of metabolic parameters.
In mammals, FGFs mediate their action via a set of four FGF receptors, FGFR1-4, that in turn are expressed in multiple spliced variants, e.g., FGFR1c, FGFR2c, FGFR3c and FGFR4. Each FGF receptor contains an intracellular tyrosine kinase domain that is activated upon ligand binding, leading to downstream signaling pathways involving MAPKs (Erk1/2), RAFT, AKT1 and STATs. (Kharitonenkov et al., (2008) BioDrugs 22:37-44). Several reports suggested that the “c”-reporter splice variants of FGFR1-3 exhibit specific affinity to β-Klotho and could act as endogenous receptor for FGF21 (Kurosu et al., (2007) J. Biol. Chem. 282:26687-26695); Ogawa et al., (2007) Proc. Natl. Acad. Sci. USA 104:7432-7437); Kharitonenkov et al., (2008) J. Cell Physiol. 215:1-7). In the liver, which abundantly expresses both β-Klotho and FGFR4, FGF21 does not induce phosphorylation of MAPK albeit the strong binding of FGF21 to the β-Klotho-FGFR4 complex. In 3T3-L1 cells and white adipose tissue, FGFR1 is by far the most abundant receptor, and it is therefore most likely that FGF21's main functional receptors in this tissue are the β-Klotho/FGFR1c complexes.
The present disclosure provides a human (or humanized) antigen binding protein, such as a monoclonal antibody, that induces FGF21-like signaling, e.g., an agonistic antibody that mimics the function of FGF21. Such an antibody is a molecule with FGF21-like activity and selectivity but with added therapeutically desirable characteristics typical for an antibody such as protein stability, lack of immunogenicity, ease of production and long half-life in vivo.